The present invention relates to a propylene polymer composition which is suitable for injection moulding of large articles. More particularly the polypropylene composition can be injection moulded into shaped articles, for example for automotive applications, especially bumpers, which have improved surface properties, particularly a reduced occurrence of flow marks. The inventive propylene polymer compositions are especially suited for the automotive applications because they have excellent impact strength/stiffness balance, high flowability and are not susceptible to the occurrence of flow marks.
Polypropylene resins are normally crystalline arid have low impact resistance, especially at low temperatures. To improve the impact resistance, polypropylene can be blended with polyethylene or ethylene copolymers and rubber-like material, such as polyisobutylene, polybutadiene or ethylene-α-olefin based copolymers.
Normally the resins are then injection moulded into the desired articles. If the articles are relatively large, such as for example automobile bumpers, instrument panels or centre-consoles, the problem of optical irregularity arises, due to the necessary long flow paths of the resin.
Such surface defects, which are also known as flow marks, tiger stripes or flow lines, are deteriorating the surface aesthetics since they manifest, during injection moulding, as a series of alternating high and low gloss strips perpendicular to the direction of the melt flow.
Many attempts to avoid these surface defects, while keeping a good balance of other physical properties, have been made, e.g. DE 19754061. It has however been found, that either the occurrence of flow marks could not be entirely prevented, or the physical properties of the polymer compositions were unsatisfactory.
It is therefore the object of the invention, to provide a propylene polymer composition, which can be injection moulded into large articles, which articles show no flow marks and which composition simultaneously shows a good impact strength/stiffness balance.
The new compositions shall be used for injection moulding, therefore the MFR of the compositions is preferred to be ≧5 g/10 min. A Charpy notched impact strength according to ISO 179/1 eA at +23° C. of ≧10.0 kJ/m2, better of ≧12 kJ/m2 and, still better, of ≧13.0 kJ/m2 is considered as good impact strength. The minimum value for the Charpy notched impact strength according to ISO 179/1 eA at −20° C. is ≧4.0 kJ/m2. Stiffness is considered to be high with tensile moduli according to ISO 527-3 of ≧1200 MPa. Still higher values are of course more preferable. The surface quality of injection moulded parts, which is determined according to the procedure described in the experimental section, must be “excellent”, i.e. only polymer compositions which can be injection moulded without showing any flow marks, solve the problem which is underlying the present invention.